The present invention relates to a video signal encoding method and is appropriately but not exclusively applied in transforming video signals to highly efficiently encoded data.
In a conventional video signal recorder, video signals are recorded on a recording medium such as a compact disk (CD) so that the signals can be randomly searched. In video conference systems and picture telephone systems, video signals are conventionally transmitted to a remote destination. In such recording and transmitting systems, a method has been proposed wherein each picture represented by a video signal is separated into blocks of a predetermined number of pixels, and the blocks then each undergo a discrete cosine transform (DCT) to produce encoded data. The transmission capacity of a transmission line is efficiently used when sending the encoded data.
In a DCT, original picture signals which correspond to each block of a picture are formed into two dimensional data arrays and are linearly transformed about a transform axis which is appropriate to the characteristic of the original picture signals. The resultant picture data of each block, in the form of an array of coefficient data are scanned in a zigzag manner and are then compressed Run-length Huffman encoding technique. The compressed data are sent and recorded.
To produce such transformed, encoded video signals for a recording medium, such as a CD, a video signal encoder 1 as illustrated in FIG. 1 is used. In the picture data generating system 1, video signal VD is quantized by a video signal encoding circuit 2 to produce an encoded data signal D.sub.VD, which is then temporarily stored in a transmission buffer memory 3, from which the encoded data signal D.sub.VD is read out as a transmission data D.sub.TRANS at a predetermined transmission rate. The transmission data signal D.sub.TRANS is sent to a picture data recording and reproducing system 5, such as a CD player, through a transmission line 4 which constitutes a transmission system. At the same time, the transmission buffer memory 3 feeds back a data remaining signal D.sub.RM, which represents an amount of data remaining in the buffer and waiting transmission, to the video signal encoding circuit 2 through a feedback line. A quantizing step STEPG is controlled during encoder of the video signal VD (FIG. 2), so that the amount of quantized encoded data D.sub.VD does not overflow or underflow the capacity of the transmission buffer memory 3.
In the picture data recording and reproducing system 5, the transmission data signal D.sub.TRANS is decoded using an inverse run-length Huffman encoding algorithm, then inverse quantized and finally inverse discrete cosine transformed. In this manner, the original video signals are reconstructed.
In the video signal encoder 1, the quantizing circuit of the video signal encoding circuit 2 stepwise changes the quantizing step STEPG according to the amount of data remaining in the transmission buffer memory 3 so that the quantizing step STEPG becomes larger as the data occupancy rate: (data remaining)/(capacity) of transmission buffer memory 3 increases, that is, when the buffer is fairly full only coefficients which have large amplitudes are transmitted. For picture data recorded on a CD at a data rate of 1.1 Mbps, for example, fine patterns of a picture are not transmitted when the quantizing step STEPG is large, resulting in the picture not being reconstructed with excellent quality.